Reusing deep frying oil? What are "the rules"?

I looked for a thread on this topic, but was unable to find one. What are the rules of thumb for saving and reusing deep frying oil? I don't remember the source, but I read once... from a source I apparently trusted... that if oil was used to deep fry protein (i.e., fish, chicken, turkey, chicken fried steak, etc.) it shouldn't be resused. However, if used on non-proteins (potatoes, tortilla chips, tempura veggies, etc.) it can be reused.

That "rule of thumb" makes deep frying turkeys a significantly more expensive proposition, given the price of peanut oil these days. A friend recently told me he has used the same oil for frying his last five turkeys, or so... apparently without any degradation in the quality of the finished product. Given the infrequency of turkey frying occasions, I can't believe the oil didn't go rancid on its own over this length of time.

Can anyone help de-confuse me? How can one determine when the oil is spent, past its prime, or gone rancid? What rules do restaurants use? I would hate to have friends over for deep fried turkey only to find the bird to be "off tasting" after all the preparation, drama and fanfare!

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